earlier also feaver, late Old English fefor, fefer "fever, temperature of the body higher than normal," from Latin febris "fever," related to fovere "to warm, heat," probably from PIE root *dhegh- "burn" (source also of Gothic dags, Old English dæg "day," originally "the heat;" Greek tephra "ashes"); but some suggest a reduplication of a root represented by Sanskrit *bhur- "to be restless."

The Latin word was adopted into most Germanic languages (German Fieber, Swedish feber, Danish feber), but not in Dutch. English spelling influenced by Old French fievre. Alternative to Old English hrið, hriðing (cognate with Old High German hritto, Irish crith, Welsh cryd, Lithuanian skriečiù). Extended sense of "intense nervous excitement" is from 1580s. Also as a verb in Old English, feferian.